Hydraulic fluid, e.g., oil, in a transmission typically has a low temperature, e.g., −10° C., under cold start conditions. At such a low temperature, there is increased flow resistance owing to an increased viscosity of the oil. As a result, the magnitude of the load imposed on the engine is substantially great. Furthermore, stroking a variable displacement pump in the transmission to maximum displacement under such conditions can damage the pump. Specifically, because maximum displacement from the pump is required to engage the clutches, at zero ground speed the machine cannot move until the oil is warm enough to stroke to maximum displacement.
Methods for warming oil in a transmission under cold start conditions are well known. One method of warming the oil in a transmission is described in U.S. Pat. No. 5,115,694 (the '694 patent) issued to Sasaki et al. The '694 patent describes a warming-up operation for an engine in a transmission that starts in cold conditions. Hydraulic fluid, e.g., oil, is supplied from a pump driven by the engine to a hydraulically actuatable coupling of the transmission, i.e., clutches and brakes, for varying a gear ratio of the transmission. If the temperature of the oil in the transmission is below a predetermined threshold, then the hydraulic pressure of the oil supplied by the pump is decreased temporarily to a minimum value. Since the magnitude of the line pressure is temporarily decreased, the load on the engine imposed by the pump is decreased and the oil may be subsequently warmed up. After temporarily decreasing the line pressure to warm the oil, the line pressure may be increased to a maximum value.
Although the warm-up strategy of the '694 patent may warm the oil in the transmission under cold start conditions, the '694 patent warms the oil by decreasing line pressure in the oil supply, thereby decreasing the amount of oil circulating in the transmission.
The disclosed system is directed to overcoming one or more of the problems set forth above.